In the case of writing and/or reading of a disk of the optical type, it is necessary:
to gain access to one zone of the disk, PA1 to remain in said zone with good precision both radially and perpendicularly to the surface of the disk (focusing).
In devices of the prior art, an "optical head" satisfies both of the functions just mentioned and is usually composed of an actuator or motor of the so-called "loudspeaker voice coil" type which permits focusing motion at right angles to the face of the disk. The optical head also comprises an actuator which serves to carry out the radial tracking motion and consists either of an oscillating mirror or of a translational-motion device which can be fixed in the moving portion of the focusing actuator.
In regard to access, this is obtained by displacing the entire "optical head" either by means of a system consisting of a screw and direct-current motor or stepping motor or by means of a motor and belt system. The linear motor technique employed in certain types of magnetic disks may also be adopted.
It is an advantage to have the possibility of employing the access actuator for carrying out radial tracking at least to a partial extent while maintaining a small overall size of actuator as well as a low value of control power.
"Optical heads" in standard use comprise one or a number of magnetic circuits for permitting focusing and radial tracking movements. This results, however, in heads on relatively substantial weight (of the order of 20 to 40 g).
Should it be desired to obtain at least partial correction of radial tracking with the access actuator, this latter can undoubtedly be only of the linear motor type. The weight of the optical head governs the dimensions and weight of the access linear motor, and consequently the control power of the access actuator.